syscall/js: add package

This commit adds the syscall/js package, which is used by the wasm
architecture to access the WebAssembly host environment (and the
operating system through it). Currently, web browsers and Node.js
are supported hosts, which is why the API is based on JavaScript APIs.
There is no common API standardized in the WebAssembly ecosystem yet.

This package is experimental. Its current scope is only to allow
tests to run, but not yet to provide a comprehensive API for users.

Updates #18892

Change-Id: I236ea10a70d95cdd50562212f2c18c3db5009230
Reviewed-on: https://go-review.googlesource.com/109195


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>